A well-informed author... NOT!
> we can't say where, as they operate out of a number of Post Office boxes.
Of course not, because the only way to discover the delivery address* for a PO Box number would be...
... to ring up the Royal Mail Business Centre for the area in which the PO Box is, and ask them. Anonymously. Period. That's it. The Royal Mail doesn't provide accommodation addresses (for which you used to have to have a licence under section 5 of the Official Secrets Act 1920, until it was repealed by the Postal Services Act 2000... yes, really!), so ANYONE can ring up without giving their name and just ask.
PO Box numbers are provided solely for pre-sorting mail, NOT for true anonymity.
If for any reason (e.g. you speak to a numpty who doesn't know the Royal Mail's own rules), just politely tell them that you are happy to hold for as long as it takes whilst said numpty consults a manager at the nearest Royal Mail Operations Centre... and as if by magic...!
Jeez... hardly cutting-edge journalism, is it?
* But not the name of the person to which it is delivered, because of... (all together now!)... the Data Protection Act!